﴿ ٱلَّذِينَ ءَاتَيْنَٰهُمُ ٱلْكِتَٰبَ مِن قَبْلِهِۦ هُم بِهِۦ يُؤْمِنُونَ ﴾

“As for those unto whom We have vouchsafed revelation aforetime - they [are bound to] believe in this one [as well];”

Those to whom We gave the Scripture before this that is before the Qur’ān they believe in it too. This was revealed regarding certain Jews who became Muslims such as ‘Abd Allāh b. Salām and others and certain Christians who had come from Abyssinia and Syria who also became Muslims.